1. Technical Field
The present invention relates to an electro-optical device, to an electronic apparatus, and to an illuminating device. In particular, the invention relates to a structure of an electro-optical device equipped with condensing layers such as prism sheets.
2. Related Art
In general, as shown in FIG. 8, a liquid crystal display device 10 is known which has a liquid crystal display panel 2 that modulates transmitted light, and a backlight 4 disposed at the backside of the liquid crystal display panel 2 (opposite to the viewer's side) that provides a light beam to the liquid crystal display panel 2 so that it realizes a desired display scheme by controlling the transmission state of the illuminating light emitted from the backlight 4 by way of a light shutter function of the liquid crystal display panel.
In the above-mentioned liquid crystal display device 10, the display brightness is often enhanced by disposing two condensing sheets 6 and 7 made of prism sheets, etc., between the liquid crystal display panel 2 and the backlight 4 to raise the percentage of the illuminating light used as the display light, i.e., the utilization efficiency of the light. On the condensing sheets 6 and 7, in order to collect the emitted light from the backlight 4 to the viewer's side, a plurality of condensing structures, more specifically, prism structures 6a and 7a made of hill-shaped or V-groove-shaped surface structures are periodically formed. Usually, the condensing sheets 6 and 7 are disposed so that the arrangement directions of the optical structural periods are orthogonal to each other.
The above-mentioned condensing sheets 6 and 7 are capable of collecting the illuminating light emitted from the backlight 4 toward the viewer's direction, thereby enhancing the brightness of the liquid crystal display device 10 by more than several tens percent. Although condensing sheets having various condensing structures are known, the condensing sheets having the hill-shaped or V-groove-shaped prism structure as the illustrated example are most common. FIG. 9 is a graph showing the relationship, for such condensing sheets (prism sheets), between the optical structural period (hill-shaped or V-groove-shaped formation period) and the rate of increase in the brightness by the condensing sheets. Here, the abscissa shows the optical structural period, and the ordinance shows the rate of increase in the brightness. Furthermore, the rate of increase in the brightness on the ordinance is shown by taking the brightness when only a diffusing sheet is disposed on the light-guiding plate of the backlight as a reference, and showing, with respect to the reference, the rate of increase in the brightness when an additional condensing sheet is disposed on the diffusing sheet. According to the graph, if the optical structural period of the condensing sheet is equal to or above 20 μm, the rate of increase in the brightness becomes 1.5 times or larger.
However, when the condensing sheets are used as described above for the liquid crystal display device 10, based on the relationship between the optical structural periods of the condensing sheets 6 and 7 and the arrangement period of the pixels in the liquid crystal display panel 2, a stripe-shaped contrast unevenness having a strong modulation degree (hereinafter, referred to as the “moiré fringes”) may occur, and thus the display quality of the liquid crystal display device may be degraded, which is problematic. To solve the problem, it is conceivable to provide a liquid crystal display device in which the optical structural periods of the condensing sheets 6 and 7 are smaller than the arrangement pitch of the pixels of the liquid crystal display panel 2 (for example, refer to JP-A-06-118410 or JP-A-08-036179).
More specifically, in this liquid crystal display device, when the pitch of the stripe grooves of the prism plate disposed between the liquid crystal display panel and the backlight is denoted as λ1, and the pitch of the transparent electrode wiring lines of the liquid crystal display panel is denoted as λ2, by making λ1≦0.075 λ2/(λ2+0.075), the gap between the moiré fringes becomes equal to or below 75 μm, and thus the moiré fringes cannot be recognized by naked eyes (refer to JP-A-06-118410).
Further, it is conceivable to provide a liquid crystal display device in which two condensing sheets are disposed between the liquid crystal display element and the backlight, and the optical structural period of the condensing sheet which is closer to the liquid crystal display element is made smaller than the arrangement period of the pixels of the liquid crystal display element, thereby obtaining a uniform display which is free of the moiré fringes (refer to JP-A-08-036179).
However, due to the reduction of the arrangement pitch of pixels together with the enhancement of precision of the liquid crystal display device these days, the arrangement pitch of the pixels approaches the optical structural periods of the commonly used condensing sheets, and thus the moiré fringes become easy to occur, and the moiré fringes becomes even more prominent, and thus the display quality gets degraded, which is problematic.
Under the circumstances, if the optical structural periods of the condensing sheets are made equal to or below the arrangement period of the pixels as in the above-mentioned liquid crystal display devices (refer to refer to JP-A-06-118410 or JP-A-08-036179), since it is necessary to use condensing sheets having fine optical structures, there is a problem that the cost to obtain the condensing sheets rises. Also, there is a problem that reducing the optical structural periods of the condensing sheets would lower the rate of increase in the brightness as shown in FIG. 9. In particular, since the rate of increase of the brightness significantly drops when the optical structural period of a condensing sheet becomes equal to or below 10 μm, using the condensing sheet itself may become meaningless.